The general concept that external magnetic fields exert forces on electric currents flowing within such fields historically has been employed in the design of electromagnetic apparatus such as electric motors. The most common type of electric motor is of the rotating type, wherein current-carrying elements and permanent magnets are configured in a manner so that forces produced by electromagnetic effects are exerted on a rotatably supported axial device, commonly referred to as an armature. The vector sum of the generated forces has a time-variant direction so as to apply a torque to the armature and rotate the same.
Another type of electric motor employing the same general electromagnetic concepts is commonly referred to as a linear motor. In contrast to an electric motor of the rotating type, the armature of a linear motor is mounted for linear movement along its axis. Correspondingly, the current-carrying elements and permanent magnets are configured so as to exert forces in the appropriate direction to produce linear armature movement. It should be noted that the term "armature" is being used herein in its broadcast sense to refer to the moving component of an electric motor, and should not be construed as referring only to rotating components.
In known linear motors, the configuration of the electromagnetic elements is usually relatively simple compared to the more complex configuration of elements typically found in rotating-type motors. For example, the general structure of several well-known linear motors includes a stationary frame or housing having an inner spacial area with permanent magnets positioned within the spacial area. The armature, often referred to as an "actuator" in linear motor terminology, can include a coiled wire or similar current-carrying element received within the spacial area and positioned between the magnets and a low reluctance element centrally located in the spacial area. The centrally located element is commonly referred to as a "pole piece". As electric current is applied through the coiled wire in the presence of the magnetic flux resulting from the permanent magnets, forces are exerted on the coiled wire of the armature. The magnets and current-carrying coiled wire are configured so that the forces are applied in a constant direction to move the armature axially relative to the pole piece.
An example of a linear motor as described above is disclosed in the U.S. Pat. No. 3,743,870 to Hunt, issued July 3, 1973. The Hunt patent describes a linear motor comprising a rectangular housing forming a pole piece, a rectangular core rigidly mounted within a spacial area within the housing to form another pole piece, and rectangular slab-like magnets mounted to the housing so as to form air gaps between the magnets and the core. A rectangular coil is supported by axially projecting stems extending out of the front of the housing. The coil axially slides within the air gaps between the magnets and the coil, with forces applied to the coil by means of application of electric current.
Linear motors have historically been utilized in applications requiring positioning control, i.e. applications where servo systems are employed. Such motors have found a particularly significant application in the emerging computer industry. Specifically, one type of data storage device now commonly employed in both large scale and small scale computer systems is the disk. Although various types of disk storage devices are now employed, a conventional type of disk consists of a flat and circular rotating surface coated with a magnetic material, such as iron oxide. Surface positions of the disk are characterized as data storage cells and alignment of the magnetic material in each cell in either of two predetermined magnetic configurations provides a binary information digit. The cells are usually arranged in concentric "tracks" on the disk, and the electromechanical assembly for rotating the disk is commonly referred to as a disk drive.
Data is typically stored on and retrieved from disks by means of movable electromagnetic transducers known as read/write "heads". The magnetic configurations of the disk data cells are imprinted or modified when the head is in a "write" mode. Sensing of the magnetic configurations is accomplished when the head is in a "read" mode. The disks are typically rotated extremely rapidly, and data can be quickly imprinted on or sensed from any cell on the disk by moving the head in a radial direction across the magnetized disk surface. Linear electric motors are commonly employed to actuate and control movement of the read/write disk heads.
Several inherent problems must be overcome when employing linear electric motors to achieve read/write functions of computer data storage devices. For example, the static performance criteria for such read/write heads includes the requirement that the heads must be held stationary over the disk surface in an extremely rigid and accurate manner in view of the extremely small size of the storage data cells.
In addition, relatively stringent dynamic performance criteria must also be met. Again, with the extremely small size of the storage data cells, substantial accuracy must be achieved in properly positioning the read/write head at a particular location adjacent the disk surface. Furthermore, a computer system cannot be substantially limited in its instruction execution time by data access time requirements. Accordingly, movement of the head across the disk surface must be rapid.
The aforedisclosed problems of stringent static and dynamic performance criteria for linear motors can be associated with several types of motor applications. However, other problems are more specifically associated with the use of such linear motors to achieve movement of read/write transducer heads for computer disks. For example, any type of electromagnetic motor generates magnetic fields in various spacial configurations. The generated magnetic fields external from the flux resulting in forces applied to the armature are commonly referred to as "stray" magnetic fields or flux. In many applications involving linear motors, the stray magnetic flux results in no apparent degradation or harm relating to the use of the motor. However, when a linear motor is utilized to control movement of a read/write head across a disk surface, the stray flux can be a substantial problem in view of the magnetic characteristics of the disks themselves. It is apparent that stray magnetic fields can readily result in erroneous magnetizations of the disk data cells. In order to overcome this problem, the motors have been shielded and spaced from the disk through mechanical linkages.
Various other types of problems are also commonly associated with various types of linear motor applications. For example, with the use of such motors in applications associated with computer disk apparatus, the configuration of the structural and magnetic elements of the linear motor should allow for the motor to be relatively small, while still providing sufficient forces and rapid response to armature current. In addition, if power consumption associated with energizing the current-carrying elements of the motor and applying forces to the armature can be maintained at a minimum, external circuit components associated with application of power to the motor can correspondingly be reduced in size.
One primary consideration with respect to the use of linear motors in substantially all types of applications relates to the physically realizable linear characteristics of the motor. That is, an ideal linear electric motor will generate forces directly proportional to the magnitude of electric current flowing in the armature coil in the presence of constant state external magnetic fields. However, it is known that linear motors employing elongated permanent magnets having a linear polarization will often exhibit some nonlinearity of generated forces. That is, even with a current of constant magnitude applied through the armature coil, the force applied to the armature at one position along its stroke may have a magnitude substantally different from forces generated at other armature stroke positions. Accordingly, the motor tends to exhibit "hot" and "cold" positions at which the generated forces tend to differ. When there is a differential in generated forces along the stroke length of the armature, the coil current required to initiate movement of the armature and to move the armature at a particular velocity or rate of acceleration can be dependent upon the relative position of the armature along its stroke. In such event, accurately controlling armature movement can be substantially difficult.
In addition, if the magnitude of generated forces necessary to move the armature are dependent upon the position of the armature along its stroke, power can be wasted. For example, if a particular location along the armature stroke length is a "cold" spot, the magnitude of current necessary to move the armature away from this stroke position may be substantially greater than the requisite current to move the armature away from other stroke positions. However, it is necessary for the engineer to design the armature current requirements on the basis of locations along the stroke length at which maximum current is required. These armature current requirements result in wasted power.
Various types of magnetic and structural configurations have been developed in attempts to overcome one or more of the previously described problems associated with linear motors and their applications. For example, permanent magnets having a radial polarization, instead of conventional linear polarization, have been utilized in linear motors to reduce stray magnetic flux. The previously referenced Hunt Patent discloses a linear motor employing radially polarized magnets.
As another example, the U.S. Pat. No. 3,723,780, to Gillum, issued Mar. 27, 1973, discloses a linear motor having a tubular and radially polarized magnet with an iron ring positioned in the internal spacial area of the magnet. A coil is disposed in the air gap between the ring and magnet. The Gillum motor also includes washer slugs and a central iron bar in contact with the ring so as to form a closed-loop low reluctance path of magnetic flux.
In another example, the U.S. Pat. No. 3,723,779, to Gillum, issued Mar. 27, 1973, describes a linear motor having a compensation winding mounted to the exterior of a center core. The purpose of the compensation winding is to overcome the effects of the magnetic field generated by the electric current in the armature coil. The magnetic flux resulting from the armature coil current tends to distort the magnetic forces exerted by the permanent magnets. Compensation winding arrangements can be particularly necessary when relatively large pole pieces are employed in the motor. In such event, the magnetic fields resulting from the armature coil current tend to be relatively large.
Another patent showing employment of radially polarized tubular magnets within a tubular housing is the U.S. Pat. No. 3,681,630, to Sutton issued Aug. 1, 1972. Radially polarized magnets having another structural configuration are shown in the U.S. Pat. No. 4,217,507, to Jaffe et al. issued Aug. 12, 1980. Jaffe et al disclose a linear motor for a sewing machine having a tubular housing and a spool of plastic material in which is embedded a series of permanent bar magnets made of semarium cobalt. A coil is wound around the spool and an armature driven by coil current reciprocally slides within the central opening of the spool.